L-carnitine(3-hydroxy-4-trimethylaminobutyrate) generally exists in organisms, and is a zwitterionic compound that carries long-chain activated fatty acids into the mitochondrial matrix across the inner mitochondrial membranes in the mitochondria. It is known that L-carnitine in the human body is synthesized from lysine or protein lysine. Generally, in a mammal, protein lysine is used as a precursor of L-carnitine biosynthesis, but free lysine is used in Neurospora crassa. In L-carnitine biosynthesis, ε-N,N,N-trimethyllysine, ε-N,N,N-trimethyl-β-hydroxyllysine, a N,N,N-trimethylamino butyraldehyde intermediate, and γ-butyrobetaine are produced, and γ-butyrobetaine is hydroxylated by γ-butyrobetaine hydroxylase to be L-carnitine. FIG. 1 is a flowchart illustrating a supposed biosynthetic pathway of L-carnitine in Neurospora crassa. 
L-carnitine can be produced by a chemical synthesis method, a semi-synthesis method using an enzyme reaction, and a method of using a microorganism. However, when the chemical synthesis method is used, there is a problem in that a racemate of DL-carnitine is obtained, and thus this has to be separated. As an example of the semi-synthesis method using an enzyme reaction, U.S. Pat. No. 4,221,869 discloses a method of producing L-carnitine from dehydrocarnitine with carnitine dehydrogenase (EC 1.1.1.108) that uses NAD as a coenzyme. However, dehydrocarnitine is very unstable, and spontaneously decomposes into acetonyl trimethylammonium and carbon dioxide. In addition, DE Patent No. DE-OS-3123975 discloses a method of producing L-carnitine from γ-butyrobetaine with γ-butyrobetaine hydroxylase (EC 1.14.11.1) separated from Neurospora crassa. However, there is a disadvantage in that α-ketoglutarate and a reductant (that is, ascorbate) should be added to a reactant during hydroxylation.
As a method of producing L-carnitine using a microorganism, for example, U.S. Pat. No. 5,028,538 discloses a method of collecting L-carnitine from the culture obtained after E. coli 044 K 74 is cultured in a medium containing crotonobetaine (4-N,N,N-triethylamino crotonic acid). In addition, U.S. Pat. No. 4,708,936 discloses a method of producing L-carnitine culturing Achromobacter xylosoxydans DSM 3225 (HK 1331b) in a medium containing crotonobetain and/or γ-butyrobetaine. However, there are disadvantages in that a precursor of L-carnitine biosynthesis, such as crotonobetain, or a compound that is not an intermediate should be used, and production efficiency of L-carnitine is not high. Therefore, there still remains a need for improving production efficiency in a method of producing L-carnitine using a microorganism.
The inventors of the present invention have tried to produce a microorganism of L-carnitine that uses an inexpensive precursor and also has a high production efficiency, and have found that genes associated with L-carnitine biosynthesis derived from Neurospora crassa were well expressed in a microorganism of Enterobacteriacae, thereby completing the present invention.